1. Field of the Invention
This invention relates to current references and switched capacitor circuits in general and, more particularly, to substantially temperature independent MOS current reference.
2. Description of the Prior Art
Implementing temperature independent current references in bipolar technology is well known. See "Analog Integrated Circuits", 2nd edition, by Gray and Meyer, pp. 284-296. However, for MOS circuits where bipolar transistors are not available or undesired, current references are more difficult to make with a low temperature coefficient. The most common form of current reference mirrors the current through a resistor coupled to a relatively temperature independent voltage reference (e.g., a bandgap reference). See pages 730-737 of the above-mentioned reference. The temperature dependence of the current through the resistor is then substantially determined by the temperature coefficient of the resistor. If the resistor is an integrated circuit resistor, the temperature coefficient thereof can be very substantial: about -2000 ppm/.degree. C. or so. This may be intolerable in certain applications. Thus, either the resistor is made to be off-chip (thereby having a well-defined temperature coefficient) or the temperature coefficient of the voltage reference is designed to partially offset the temperature coefficient of the resistor. In either case, the result may be impractical or not of sufficient tolerance for the desired application.
It is therefore desirable to have a MOS current source with a low temperature coefficient that does not rely solely on a resistor for temperature stability.
It is also desirable for the low temperature coefficient current reference to be implementable solely in an integrated circuit.